A patient with anti-Coa having activated monocytes specifically phagocytosing IAT-negative Co(a+) red blood cells.

Background: The antigen, Co^a(CO1), is a high prevalence antigen. There is not a substantial amount of evidence regarding the clinical significance of anti-Co^a, especially if this antibody can cause phagocytosis. Using the monocyte monolayer assay (MMA), we have investigated the potential clinical significance of anti-Co^a in a patient that appeared to have an activated mononuclear phagocyte system.

Study design and methods: A female, AB RhD-negative patient having anti-Co^a was investigated for the potential clinical significance of the antibody using the MMA. On initial presentation, MMA using both autologous and allogeneic monocytes were performed using the following red blood cell (RBC) samples: four Co(a+), one Co(a-) autologous RBC, a positive control of RhD-positive RBC opsonized with anti-D and a nonopsonized RhD-negative control. The patient's plasma was tested against all RBC samples by a saline-IAT (sIAT). A DAT on the patient's RBC was performed using polyspecific and monospecific antihuman globulin sera The autologous MMA was performed initially on presentation and again, 2 months later.

Results: Initial autologous MMA showed significant phagocytosis with all 4 Co(a+) donors, despite the sIAT being nonreactive with both IgG and C3d. The RBCs from the Co(a-) donor did not show any phagocytosis. The patient had a negative tube DAT, however, was able to significantly phagocytize their own RBCs. Anti-D-opsonized RhD-positive RBCs showed greatly enhanced phagocytosis compared to what is usually observed. Allogeneic MMA showed no significant phagocytosis. Follow-up autologous MMA, 2 months later, when the patient was stable having a hemoglobin of 15.5 g/dL, showed similar results as the initial MMA using the dame donor RBCs as in initial testing; however, the patient's monocytes no longer gave a positive result with autologous RBCs.

Discussion: These results suggest an enhanced activation of the patient's monocytes that is specific for Co(a+) RBCs and indicate the anti-Co^a as potentially clinically significant.